The goal is to ascertain by immunochemical methods if specific types of cancer can be detected and identified by measurement of RNases presumably released by malignant tissues. Enzymatic and radioimmunological assay procedures have been used to measure levels of pancreaticlike ribonuclease in serum from several hundred individuals. The level of pancreatic-like RNase in healthy blood donors or other volunteers was found to be age and sex dependent. At any age serum RNase in males is likely to be greater than in females of the same age, and in both sexes levels rise gradually with age. Assays of serum samples taken from any single individual over approximately a 2-year period show a very stable RNase level, with no measurable variation. Immunologically measured RNase levels in hospital and clinic patients generally show more variability, with some values significantly above normal. In pancreatic carcinoma about a half of these patients showed elevated pancreatic-like RNase, while the rest were at or near the expected levels for their age and sex. Other types of cancer resulted in fewer elevated levels (e.g., stomach or gastrointestinal cancer, lung cancer) or in a similar scattering (e.g., breast cancer). Only in kidney and bladder cancer patients was the RNase level almost always high. Preliminary results indicate that the high RNase level is also characteristic of nonmalignant pancreatic diseases and roughly correlated with kidney function. Our conclusion at this point must be that serum RNase levels, whether measured by enzymic assays or immunologically, cannot be used as a specific biochemical marker of any particular disease. But RNase still may be a marker of disease progress or therapy, or a sensitive marker of kidney function.